In the technical field of positron emission tomography (hereinafter abbreviated to PET) and radiation detection and measurement, a scintillator detector is commonly used for energetic particle detection. The general operating principle of the scintillator detector is that: energetic particles are converted into visible light by a scintillation crystal, and then the visible light is converted into an electrical pulse signal by a photodetector. A typical electrical pulse signal is shown in FIG. 1. The electrical pulse signal may be further processed to acquire information of the energetic particles, such as energy and time information.
The acquiring time information where the electrical pulse passes over a threshold is widely applied in the field. In conventional methods, the time information of the energetic particle is acquired by detecting a time point when the electrical pulse passes over a preset threshold. Meanwhile, a multi-threshold sampling device in the conventional technology is used to implement electrical pulse digitization by acquiring the time information where the electrical pulse passes over multiple thresholds, so as to solve the technical issue of insufficient sampling rate and high power consumption of the traditional analog-to-digital converter (hereinafter abbreviated to ADC) while digitizing such electrical pulses.
Currently, the detection of the time points when the electrical pulse passes over the threshold is implemented by combining a comparator with a time-to-digit converter. The comparator is configured to compare the electrical pulse with the threshold and output high-level signals or low-level signals based on the comparison result. Ideally, the time point when the electrical pulse passes over the threshold corresponds to a transition between a high level and a low level in the outputted signals of the comparator, and the time point for the transition may be captured with the time-to-digit converter. In practice, multiple transitions may occur in the outputted signals of the comparator during a time period where the electrical pulse passes over the threshold due to noises in the electrical pulse. As shown in FIG. 2, in a falling edge of a scintillation pulse, during a time period where the scintillation pulse passes over the threshold, multiple transitions occur in the output signals of the comparator during a time period from B to C due to the influence of the noises, where B is a time point for a first transition and C is a time point for a last transition. Apparently, an accurate time point when the scintillation pulse passes over the threshold should be between B and C. Conventionally, the time point for the first transition, i.e., the time point B shown in FIG. 2, is captured as the time point when the scintillation pulse passes over the threshold. The method may be implemented with a simpler circuit, but the acquired time point is not sufficiently accurate.
Therefore, in light of the above technical issue, there is a need to provide a new method for acquiring a time point when a scintillation pulse passes over a threshold, so as to solve the issue that the time points can not be acquired accurately due to noises, and thus provide more accurate time points when the scintillation pulse passes over the threshold.